service worker: Queue Client.postMessage() until onmessage is set.

This is a follow-up to
https://chromium-review.googlesource.com/c/chromium/src/+/1441657.

This enables the queue when onmessage is assigned to. It adheres to
the specification at
https://w3c.github.io/ServiceWorker/#dom-serviceworkercontainer-onmessage:
"The first time the context object’s onmessage IDL attribute is set, its
client message queue must be enabled."

This makes the test cases about "setting onmessage" in
the WPT postmessage-to-client-message-queue.https.html pass, but the
test file will still time out until startMessages() is implemented.

Bug: 915685
Change-Id: I4c4930b48df68ee351676e5c3edfb28969fe3610
Reviewed-on: https://chromium-review.googlesource.com/c/1441734
Commit-Queue: Matt Falkenhagen <falken@chromium.org>
Reviewed-by: Hiroki Nakagawa <nhiroki@chromium.org>
Cr-Commit-Position: refs/heads/master@{#626929}

third_party/blink/renderer/modules/service_worker/service_worker_container.cc

@@ -557,6 +557,18 @@ const AtomicString& ServiceWorkerContainer::InterfaceName() const {
   return event_target_names::kServiceWorkerContainer;
 }
 
+void ServiceWorkerContainer::setOnmessage(EventListener* listener) {
+  SetAttributeEventListener(event_type_names::kMessage, listener);
+  // https://w3c.github.io/ServiceWorker/#dom-serviceworkercontainer-onmessage:
+  // "The first time the context object’s onmessage IDL attribute is set, its
+  // client message queue must be enabled."
+  EnableClientMessageQueue();
+}
+
+EventListener* ServiceWorkerContainer::onmessage() {
+  return GetAttributeEventListener(event_type_names::kMessage);
+}
+
 ServiceWorkerRegistration*
 ServiceWorkerContainer::GetOrCreateServiceWorkerRegistration(
     WebServiceWorkerRegistrationObjectInfo info) {
@@ -600,6 +612,10 @@ ServiceWorkerContainer::CreateReadyProperty() {
 
 void ServiceWorkerContainer::EnableClientMessageQueue() {
   dom_content_loaded_observer_ = nullptr;
+  if (is_client_message_queue_enabled_) {
+    DCHECK(queued_messages_.empty());
+    return;
+  }
   is_client_message_queue_enabled_ = true;
   std::vector<std::unique_ptr<MessageFromServiceWorker>> messages;
   messages.swap(queued_messages_);

third_party/blink/renderer/modules/service_worker/service_worker_container.h

@@ -103,9 +103,9 @@ class MODULES_EXPORT ServiceWorkerContainer final
   const AtomicString& InterfaceName() const override;
 
   DEFINE_ATTRIBUTE_EVENT_LISTENER(controllerchange, kControllerchange);
-  // TODO(falken): Enable the client message queue when onmessage is assigned
-  // to.
-  DEFINE_ATTRIBUTE_EVENT_LISTENER(message, kMessage);
+
+  void setOnmessage(EventListener* listener);
+  EventListener* onmessage();
 
   // Returns the ServiceWorkerRegistration object described by the given info.
   // Creates a new object if needed, or else returns the existing one.